The removal of liquids which accumulate in producing wells is required in order to enhance production from the well and the overall operation of the production system. In particular, liquids removal is necessary for the dewatering of gas wells and the removal of oil from wells where mixed oil and gas exists in the underground reservoir. If the liquids, such as water and/or oil, are not removed, the liquids tend to accumulate and fill or load up the well, which restricts the flow of the gas to the surface. Eventually, the liquids will choke off gas production completely. Therefore, the problem to be overcome is to remove the liquids continually to avoid their accumulation in the well.
One approach to this problem is to use a gas lift system which uses the natural gas pressure in the reservoir to lift the liquids from the well. In a gas lift system, a tubing string is typically located in the well which extends from the surface into the accumulated liquids such that the accumulated liquids may flow into the tubing string. The gas then enters the tubing string from the underground reservoir at chosen intervals along the tubing string to cause the liquids within the tubing string to rise to the surface. A freely moveable plunger or pig may be located in the tubing string to minimize the penetration of the gas through the liquids. Where the gas lift system uses the pressurized gas from the reservoir to transport slugs of the liquid to the surface, a small diameter tubing string for producing the liquids is often required so that the gas pressure and the gas velocity are sufficient to carry the liquids to the surface. However, the requirement for small diameter tubing may significantly restrict the flow of the liquids and reduce the gas production. As well, the gas and the liquids are typically well mixed at the surface, which may cause problems in surface production lines, such as hydrate formation or freezing. Further, gas lift systems have been found to be unsuitable where the downhole gas pressure or the gas velocity is low and thus, the gas is unable to overcome the pressure head of the liquids to carry the liquids to the surface.
Further gas lift systems have been designed which only periodically or intermittently lift the liquids to the surface in a cyclical operation in order to allow the natural gas pressure to develop in the well between the cycles to a critical level necessary to lift the liquids. Examples of such systems include U.S. Pat. No. 2,136,229 issued May 16, 1938 to Baldwin et al, U.S. Pat. No. 4,596,516 issued Jun. 24, 1986 to Scott et al and U.S. Pat. No. 4,465,435 issued Aug. 14, 1984 to Copas. Some of these systems use a timer operated valve, located in the outlet of the tubing containing the liquids. The valve is set to periodically open at a timed interval equal to the time required for the natural gas pressure in the well to recover following the release of such pressure. Other systems use valves sensitive to a predetermined differential pressure between the liquids in the tubing string and the gas to control the periodic opening of the valve to allow lifting of the liquids by the gas.
Other gas lift systems introduce pressurized gas into the well from an outside source in addition to the natural gas from the reservoir, as shown in U.S. Pat. No. 2,132,738 issued Oct. 11, 1938 to Knox. However, the introduction of the pressurized gas into the well to lift the liquids requires the use of a compressor which tends to increase both the cost and complexity of the production apparatus required.
A further approach to the problem of liquid loading is shown in Canadian Patent No. 1,167,760 issued May 22, 1984 to Prather which describes a reciprocating surface pump which is powered by the natural gas pressure from the reservoir. The reciprocating surface pump is connected to a string of sucker rods which are connected to a conventional downhole pump. In essence, the gas from the well is conducted to the surface where is drives the reciprocating surface pump. The reciprocating pump then powers the downhole pump, which pumps the liquids to the surface. Several disadvantages are exhibited by this system. First, the system requires a reciprocating pump at the surface. Second, as the gas is conducted to the surface for powering the reciprocating pump, the reciprocating pump must be designed as a pressure vessel able to withstand the pressure differential between the atmosphere and the downhole pressure. As well, there may be some energy loss as the gas travels from the bottom of the well to the reciprocating surface pump. Third, reciprocation of the sucker rods within the tubing string results in wearing of the tubing string and energy loss due to friction between the sucker rods and the tubing string.
There is therefore a need in the industry for an improved apparatus and method for moving liquids contained in an underground reservoir using a wellbore, which apparatus is powered by the pressure of the gas naturally contained within the reservoir. Further, there is a need for the apparatus to be contained downhole when in operation such that the apparatus creates a mechanical advantage for pumping the liquids and directing the gases either towards the surface or towards a downhole location such as a reservoir.